Desirability of Location Information
A long standing problem in radio communications is accurately determining the location of a mobile radio transmitter. Precise location information in a cellular telephone network is desirable for various reasons that have been recognized in the prior art. These applications of location information have yet to be realized in urban environments, however, because no practical method of providing accurate location information has yet been developed.
Multipath as the Primary Problem in Location Finding
Multipath is the greatest source of error in prior art methods of location finding. As shown in FIG. 1, multipath is typically caused by the reflection of signals from objects in the environment, such as buildings, hills, and other structures. A signal transmitted from a cellular phone 30, for example, is reflected from structures 32, 34, and 36, resulting in three multipath signals arriving at a base station 38 in addition to a direct path signal. The multipath signals not only have different directions from the direct signal, but different delays as well. As a result, multipath is a problem for location-finding systems based on direction finding, and those based on time-of-arrival measurements. For example signals from phone 30 arrive at base station 38 from different directions and with different delays, with the result that three additional cellular phones 40, 42, and 44 appear to be transmitting similar signals from different directions and at different distances. In some cases, signals from a phone 50 can arrive at base station 38 from nearly opposite directions, one from actual phone 50 and another from an apparent phone 52. In urban environments, often no direct path exists and the base station receives only multipath signals. For example, phone 46 has no direct path signal to base 38. Consequently, it appears from base 38 that a unique signal is originating from an apparent phone 48 which has a very different location from actual phone 46. Clearly, multipath significantly complicates the communication of signals, and, in particular, complicates the problem of accurately determining the true location of a transmitter. Since a large proportion of cellular phone usage is in urban environments which have severe multipath, it is especially important to solve this problem. All prior art methods, however, have failed to provide consistent and accurate location information in multipath environments.
Prior Art Location Finding Techniques
As illustrated in FIG. 2, a common prior art approach to determining the position of a mobile phone 54 involves measuring temporal information, e.g. time of arrival (TOA) or time difference of arrival (TDOA), at three or more synchronized base stations 56, 58, and 60. By communicating this temporal information between the base stations over a communication line 62, the transmitter location can be determined. In more sophisticated time-based approaches, handshaking protocols are used in conjunction with time delay measurements.
These methods have the disadvantage that signals from cellular phone 54 must be received by several base stations 56, 58, and 60 at the same time. In many circumstances, however, the phone 54 is only within the range of one or two base stations. Additionally, expensive high accuracy clocks are required at the base stations and expensive high bandwidth communication lines 62 are required between the base stations in order to allow accurate temporal correlation of their received signals from phone 54. More significantly, this approach encounters serious difficulties in urban environments since there is, in most cases, no direct path between the mobile and the base station. Consequently, the multipath can introduce large temporal delays. Although spread spectrum techniques can reduce the effects of multipath to some extent, they cannot provide high accuracy in severe multipath environments or cases where there is no direct path signal. Consequently, these methods fail to determine positions accurately and consistently in many cases.
Although the prior art does disclose several techniques for location finding that attempt to mitigate multipath effects, they all fail in the presence of severe multipath and when there is no direct path signal. These techniques are all characterized by an attempt to cope with multipath by circumvention or discrimination of multipath signals from direct path signals. In cases of severe multipath, however, there often is no direct path signal at all. In such cases, these approaches fail. Moreover, averaging techniques are based on assumptions about the distribution of multipath that are not generally valid, especially in severe multipath environments. Even in cases where such assumptions do hold, these averaging techniques do not yield accurate position information.
As illustrated in FIG. 3, another prior art approach determining the location of a phone 68 makes use of antenna arrays 64 and 66 for direction finding. For example, U.S. Pat. No. 5,515,378 to Roy, III et al. discloses a method and apparatus for estimating positions and velocities of mobiles from antenna array measurements of their signals. When an estimate of location is made based only on the directional information from a single base station, such an estimate has a very poor accuracy. To obtain more accurate location estimation, the DOA parameters must be supplemented by TOA measurements and/or parameter measurements sent over a communication line 72 from other base stations. Even in this case, however, the estimates are still not sufficient to accurately determine a correct location since a direct path may not exist at all, as in severe multipath environments. For example, since no direct path exists from phone 68 to either base station 64 or 66, phone 68 will appear to be located at the location of a false phone 70.
U.S. Pat. No. 4,799,062 to Sanderford, Jr. et al. proposes an approach to location finding using a differential multipath signal technique. They point out that when the positions of two mobiles are close to each other, their multipath signals should be nearly the same. Consequently, if a reference signal from a known transmitter location near the mobile were subtracted from the mobile's signal, the multipath effects should cancel and the differential position between the two could be determined. The patent, however, does not disclose in detail how such a method might be implemented. Moreover, in severe multipath environments the approach of Sanderford fails. Since the multipath components of the signal can change significantly over distances on the order of 30 meters or less, the differential position will be accurate only in cases where the phone is already within sight of the mobile, therefore defeating the purpose of the technique. Even in cases of less severe multipath, the technique is unattractive to implement due to the need for a reference signal from a nearby transmitter having a known location.
LeBlanc et al. in U.S. Pat. No. 5,602,903 discloses a method for determining the location of a mobile by making RF measurements, such as signal strength, error rate, and signal quality, then comparing these measurements with a database of similar measurements made when the receiver was at a known location. The disclosed method, however, suffers from the disadvantage that it requires the measurement of information at multiple base stations to determine location. Because the RF measurements that are measured by the method are only weakly correlated with position, measurements from a single base station are not sufficient to determine an accurate location.
Adaptive Array Techniques
Other more recent work in mobile communications has attempted to cope with severe multipath, albeit not for location determination applications. For example, U.S. Pat. No. 5,634,199 to Gerlach et al. discloses a base station beamforming method which uses feedback from a mobile to determine a characteristic subspace of the mobile's instantaneous channel vector. Although the instantaneous channel vector can change rapidly in a strong multipath environment, Gerlach et al. point out that it is normally restricted to a characteristic subspace that is much more stable in time. By tracking this channel subspace rather than the channel vector, much lower feedback rates are required. A collection of instantaneous channel vectors are measured, and the sum of their outer products is taken to produce a channel matrix. The eigenvectors having large eigenvalues define a subspace of this matrix which is a more stable representation of the receiver's channel. This subspace is then used for downlink beamforming at the base station to minimize crosstalk and maximize the desired signal at the mobiles. Although this approach reduces the amount of feedback required for beamforming in severe multipath environments, it does not have obvious application to location finding.
CDMA and Spread Spectrum Techniques
Code division multiple access (CDMA) is a spread spectrum wireless digital communication technique that enjoys some reduction in the effects of multipath. In contrast to earlier FDMA techniques that assign users to narrow frequency channels in the band, CDMA does not limit individual users to narrow frequency channels but spreads them all throughout the frequency spectrum of the entire band. Signals sharing the band are distinguished by assigning them different pseudonoise (PN) digital code sequences. The well-known correlation receiver uses this known signal structure to decompose multipath parts, provided they are separated in time by at least one chip. The different parts can then be recombined using a RAKE receiver to improve signal strength. Although this technique helps reduce the effects of multipath on signal fading, it does not provide any location information. Because CDMA systems are becoming more widespread, there is a particular need for accurate location finding techniques in wireless communication systems based on CDMA.